"Just imagine how far you can go with a little bit of English"
The Slovenian-born former model was chosen because she was "the most recognized emigrant to the U.S. from this region," spokeswoman Ivis Buric told CNBC.
But Trump's image was used without permission. On Tuesday, the signs came down under the threat of a lawsuit by the first lady's lawyers, according to the Associated Press.
"They used Mrs. Trump's image and personality for commercial purposes without her consent, which is a breach of the advertising code and the law on obligations," White House spokeswoman Stephanie Grisham wrote in an email.
The billboards advertised a local language school whose American co-founder held "the lofty goal of bringing the American education system closer to Croatia." Fifteen weeks of an advanced general English course cost 1,960 Croatian kuna, about $315, with more expensive courses such as "English for the E.U."
The signs, which used an image of Trump from her speech during last year's Republican National Convention in Cleveland, were not meant to be political, said a spokeswoman for the school.
"People can have all sorts of opinions about Melania Trump, but they cannot deny her the success she deserves and her knowledge of English," Buric told local media, according to the Telegraph.
Trump's lawyer, Natasa Pirc-Musar, told the AP that the school had apologized for the billboards and took steps to remove them. The school did not immediate respond to a request for comment.
Others have also sought to capitalize on Trump's fame.
In neighboring Slovenia, her name and face has been used on honey jars, cakes, high heels and even a Christmas tree last December, according to the Associated Press.
Pirc-Musar told the AP last year that, "We have nothing against the Christmas tree in Ljubljana named Melania because this has nothing to do with commercial purposes. But we can't allow some products to be named 'Melania Trump.'"
It's not uncommon for the likeness of American celebrities to appear on advertisements in other countries without their consent. Actor Zach Braff recently discovered himself in a Russian poster that may be about erectile dysfunction.
The Croatian school's spokeswoman thought the message of their signs had been lost.
"We are very sorry that the billboards were misunderstood as something intended to mock the U.S. first lady," Buric said.
"It was meant to be something positive, to show her as a role model."
Even though they were up for less than a week, the billboards were "very successful," Buric said, generating coverage from both local and international media.
Born Melanija Knavs, Trump worked in Milan and Paris before she came to the United States, where she met her future husband in 1998. They married in 2005. She is the second foreign-born first lady of the United States (Louisa Adams - who was born in England and the wife of the sixth president, John Quincy Adams - was the first)
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